Minimally invasive surgery that is aimed to treat degenerative disc disease and preserve motion in the spine is currently under investigation. Since the onset of the degenerative cascade in the disc is typically associated with dehydration and volume loss in the nucleus pulposus, a potential early intervention step may involve adding mechanically suitable material to the nucleus pulposus at the first observable signs of discomfort and loss of disc height. This procedure would restore nuclear volume and pressure against the inner wall of the annulus fibrosus. In certain embodiments, a degree of decompression or “lift” between the adjacent vertebrae may be possible with this technique. In effect, the result would be the “re-inflation” of the annulus fibrosus, the annular “tire”. Desirable outcomes of the procedure would be motion preservation, pain relief, and maintenance or restoration of disc height. Long-term re-modeling of the biological allograft-based implant into fibrous tissue or disc-like tissue would also provide favorable clinical outcomes.
At present, there are no nucleus pulposus replacement devices or augmentation technologies available for clinical usage in the United States. The Prosthetic Disc Nucleus (PDN), which is manufactured by Raymedica, was the first implant designed for nucleus replacement with the intention of attempting to restore natural mechanics in the spine. This implant is an acrylic-based hydrogel encased in a polyethylene jacket. While this technology has been implanted in over 3000 patients in Europe, significant issues regarding implant migration and implant hardening have been encountered. Other drawbacks in the design of this implant include the requirement of a substantial annulotomy and total nucleotomy as well as the inability of the implant to fill the entire nuclear cavity. In addition, the limited ability of the implant to swell inside the disc nucleus leads to high extrusion rates and inadequate load transfer of compressive forces in the disc nucleus to tensile forces on the annulus fibrosus.
Generally speaking artificial disc replacements falls into two general categories, total disc replacement and nuclear replacement. Total disc replacement devices have a number of problems; namely that they are large and non-compressible, require the removal of a large portion of the annulus and require a highly invasive surgical approach in order to be implanted. If these disc replacement devices do not remain firmly attached to the vertebral bodies, these implants can extrude or migrate from their intended position, cause significant complications and are very difficult to revise. The second category of disc replacement is nuclear replacement which is a form of partial disc replacement. Various types of methods and devices have been used to attempt to accomplish successful disc replacement.
U.S. Pat. No. 6,652,593 issued Nov. 25, 2003 is directed toward an osteoinductive implant comprising demineralized cancellous bone, which comprises a non-particulate bone. A unitary bone block is compressed into a smaller configuration such as a pellet and then hardened via drying. Upon re-hydration, the pellet will expand and assume its original shape inside a cavity. The implant is capable of being compressed and hardened into a first shape and then capable of expanding into a second shape larger than the first shape when re-softened and permitted to expand. The '593 implant is designed to be supplied either in geometries that fill correspondingly sized voids or in compressed initial geometries that are used to expand and fill any given shape smaller than or equal to their expanded size.
United States Patent Publication 2006/0030948 filed Sep. 21, 2005 is directed toward an osteogenic implant having a predetermined shaped formed of an aggregate of different sized elongate (possessing a high median length to median thickness ratio) bone particles.
United States Patent Publication No. 2004/0054414 filed on Sep. 18, 2002 is directed toward a method of augmenting an intervertebral disc nucleus by the surgical addition of a particulate collagen-based material. The collagen-based material, having a mean particle size ranging from 0.05 mm to 5 mm, may be injected in either a wet or dry state and may be supplemented with growth factors, proteoglycans, and cells. The '414 publication notes the use of demineralized bone matrix particles with sizes ranging from between 0.05 mm and 3 mm and the use of elongated cylindrical plugs. The plugs are described to be dehydrated and compressed in the radial direction and are inserted into delivery cannula for delivery into the disc space. The cylindrical plugs are delivered via extrusion through a cannula, and expand upon exiting the cannula by re-hydrating in the disc space. Examples 6 and 7 refer to the design and implementation of cylindrical plugs, which can be fabricated from solid, porous, or fibrous collagen.
Additional continuing United States Published Patent Applications Nos. 2005/0197707 filed Apr. 25, 2005 and 2005/0119754 filed Jan. 6, 2005 are based on the '414 publication. The '707 publication is directed toward the use of small particles of particulate fascia lata, particulate disc annulus material, annulus fibrosis, demineralized bone matrix and collagen which are added to the nucleus of an intervertebral disc. The '754 publication covers a method of augmenting an intervertebral disc nucleus by adding a plurality of collagen-rich tissue particles having a mean particle size between 0.25 and 1.0 mm to the disc plus a biologically active substance that promotes healing, repair or regeneration of the disc. This biologically active substance is further defined to be stem cells, hyaluronic acid, or growth factors while the collagen material is stated to be potentially allograft tissue. Radio contrast material may be added to enhance imaging of the injected material.
Another United States Patent Publication No. 2005/0055094 filed Nov. 5, 2003 discloses a system for replacing a disc nucleus involving an injection tube, a volume of fibrous tissue material to fill a nuclear cavity, and an insertion device for dispensing the tissue promoting material into the disc. Suitable fibrous tissue promoting material is listed as fascia, natural and/or man made polymeric fiber, fibrous tissue inducers such as talc, pharmaceuticals and/or minerals and fibrous tissue morphogenic protein.
U.S. Pat. No. 5,571,189 issued Nov. 5, 1996 describes an expandable bag filled with biological tissue for spinal fusion but does not show motion preservation in the spine.
The present inventive disc nucleus implant is a combination of multiple units of demineralized cancellous tissue treated to be non-osteoinductive that are designed to be small uniform geometric shapes which have the ability to pack together and act mechanically as a single unit under the compression of packing and not to comprise a non-particulate portion of a bone. The inventive tissue forms are compressed upon delivery into a cavity, but only to fit into the delivery device and not into a defined shape. In addition, the inventive tissue forms do not regain their original dimensions following the completion of the implantation procedure. In fact, the appropriate mechanical properties are only achieved if the mass of units is under compression and behaving as a coherent load-bearing material. The plurality of units that constitute the inventive implant, when taken together in an uncompressed state, have a geometry that is substantially larger than the cavity into which they are placed. Thus, the inventive implant takes on a smaller size in the confined space into which it is placed. Finally, the inventive allograft tissue form is treated to be non-osteoinductive, which achieves the desired outcome of motion preservation in the spine versus spinal fusion.
The noted prior art publications cite examples of various allograft tissues for usage such as demineralized bone matrix, disc annulus, fascia, ligaments, tendons, skin, or other connective tissues. The inventive tissue implant would not be provided in a dehydrated state and will be compressed axially inside the delivery tube rather than radially.
Advantages of the present inventive approach in comparison to other techniques include its ability to be entirely performed in a minimally invasive manner, total nucleotomy is not required and the implant size is adjustable by the volume of material that is added into the pouch. If desired an expandable pouch that is intended to hold the shaped units can be inserted into the disc nucleus through a small diameter hole and it will be enlarged with implant material to a size considerably larger than the insertion hole allowing the implant dimensions to conform to the existing cavity of the disc nucleus, with the porous pouch preventing the escape of material from the nuclear space while allowing the free transfer of fluid across its surface along with potential tissue ingrowth.